Transmittance type spectrophotometric measurements are normally made by simply directing a light through a cuvette containing a fluid sample to be analyzed. A portion of the light beam is absorbed by the sample and the remaining portion passes through the cuvette to photodetector means for measurement. By comparing the measurement obtained from the sample with a measurement obtained from a control fluid the concentration of the analyzed sample can be calculated.
A major source of error in spectrophotometer systems lies in imperfections in the glass or plastic cuvette walls. These imperfections alter the nature of the light beam which passes to the photodetector means and very minor variations in a cuvette will substantially affect measurements made by the photodetector means. Distortion of the light beam passing through a cuvette means that it is impossible to obtain reproducible results. This is particularly true for spectrophotometers in which a slit or window is located adjacent to the photodetector means for the purpose of selectively regulating measurements which are made.
In order to compensate for the aforementioned problem highly accurate spectrophotometer systems utilize optically ground and polished glass or quartz to form cuvettes for holding specimens to be analyzed. These cuvettes have at least two sides which are parallel surfaces of extremely high uniformity, thereby effectively minimizing all distortion of light passing through the cuvettes. Needless to say, such cuvettes are very expensive to manufacture. For this reason accuracy is normally sacrificed by using conventional cylindrical cuvettes resembling test tubes in transmission type spectrophotometers.